When small formats can get to equivalent speeds of large formats I think people will begin to seriously reconsider. Small sensor tech is already beginning to creep. For example RX100-II is only 1 stop down in speed from the typical APS-C sensor, despite being 2 stops down in area. If you have a format that is physically 1 stop smaller than the next step up, but has cleanness parity through tech and faster glass, I do think a lot of folks would move over. For example if Nikon 1 system came out with a 1.8 zoom or 1.2 primes (for reasonable prices) I think a lot of folks would give them a look.

I disagree though that big formats will ever go away.

I didn't say they would go away. I do believe there will be less people using them though.

People still shoot MF, people will always shoot FX if for no other reason than being invested in the glass. D700, 5DII, D4 etc are all still top of the line in image quality.